There is a continuing need for rapid sensitive methods for the detection of viral diseases. Rapid detection and identification of viral pathogens is essential to limit transfer and spread of viral disease and to monitor treatment methods. The need for such methods can be illustrated be the events surrounding the emergence of SARS.
SARS is a viral respiratory illness caused by the SARS-associated coronavirus (SARS-CoV). SARS was first reported in Asia in February 2003. Over the next few months, the illness spread to more than two dozen countries in North America, South America, Europe, and Asia before the SARS global outbreak of 2003 was contained. The symptoms of SARS included high fever, dry cough, and other flu like symptoms that can progress to pneumonia. Death occurred in approximately 15% of the infected patients. The contagious nature of SARS and its fast spread in several countries in Asia caused considerable concern throughout the world.
The SARS coronavirus belongs to a group of viruses similar to those causing the common cold. The SARS virus spreads by close person-to-person contact and is thought to be transmitted most readily by respiratory droplets produced when an infected person coughs or sneezes. Droplet spread can occur when droplets from an infected person are propelled a short distance (generally up to 3 feet) through the air and deposited on the mucus membranes of nearby persons. The virus also can spread when a person touches a surface contaminated with infectious droplets and then touches his or her mouth, nose, or eyes.
In most cases, symptoms of SARS first occur 2 to 5 days after exposure to the virus. However, the incubation period may be up to 2 weeks. Because of the extended incubation period, patients infected with the SARS virus may be placed in quarantine with those who are not infected. This may cause the uninfected patients and medical staff to become exposed to the SARS virus and to develop the symptoms of SARS.
Because of its ease of spread and long incubation period, it is critical to reliably determine the presence of the virus in a patient thought to be infected with SARS. Ideally, such a test should be sufficiently sensitive to detect the virus at an early stage of infection. The test should also be specific and have a low occurrence of false-positive and false-negative results.
Without a reliable test that can be used in the early stages of SARS infection, physicians and health care teams rely on a process of elimination, ruling out other known causes of the severe pneumonia before diagnosing SARS. However, a positive test result for another respiratory pathogen does not completely rule out infection with the SARS virus. Patients can be co-infected with both the SARS virus and other respiratory pathogens.
At present, two types of tests detect the presence of the SARS virus. The first of these is an enzyme immunoassay (EIA) test which detects serum antibody to SARS. The other test is a polymer chain reaction (PCR) test which detects the viral genetic material.
During the course of infection with the SARS virus, levels of specific anti-viral antibodies rise in the blood. Many of the tests currently available for the diagnosis of SARS are based on the detection of such antibodies. Such tests are typically either Enzyme Linked Immunoassays (ELISAs) or immunofluorescence assays (IFAs). With ELISAs, the antibodies cannot be detected until about 20 days after infection. IFAs can detect antibodies approximately 10 days after the initial infection. However, such assays are comparatively slow and require the growth of the virus in a cell culture.
The utility of antibody tests for detecting viruses, such as SARS, may be further limited due to the rapid mutation rate of some viruses. A Canadian study has detected the SARS virus in only 60% of those with SARS infection. Such results suggest that the virus is unstable and is mutating rapidly. This is not unexpected as coronaviruses are notorious for changing their outer surface antigens rapidly, a process termed antigenic drift.
Techniques such as the Polymer Chain Reaction (PCR) allow direct detection of the virus genetic material and, in theory, can detect infection at a very early stage. Many PCR tests use oligonucleotide microchip technology for detecting the virus with throat swabs, sputum or feces. Such tests typically take a few hours to perform and are relatively costly. In addition, currently available PCR methods give 40% of false positive and negative results, making the method ineffective.
Because of the deficiencies of presently available testing methods, there is a need for an improved test enabling the presence of viruses, such as the SARS virus, to be accurately detected at an early stage of infection. Such a test will benefit those showing symptoms of SARS by allowing for the monitoring of the course of their infection and subsequent recovery. In addition, a quick and effective test will benefit persons suspected of having the disease by allowing uninfected persons to be released from quarantine.
There is also the need for an automated test avoiding the need for manual intervention. Such a test will prevent spread of the disease due to infection during the testing process.